1. Field of the Invention
The present invention relates to a nitride-type III-V group compound semiconductor device, and in particular, to a semiconductor device using two-dimensional electron gas which has outstanding operational characteristics at high output, high frequency, and high temperature.
2. Description of the Related Art
Semiconductor devices using two-dimensional electron gas include a hetero-structure field-effect transistor (HFET), a high-electron-mobility transistor (HEMT), and a modulation-doped field effect transistor (MODFET). As such semiconductor devices using two-dimensional electron gas, a device using GaAs-type materials is under development.
As shown in FIG. 8, a conventional GaAs-type HFET generally includes, on a semi-insulating (SI-) GaAs substrate 101, an undoped GaAs buffer layer 102 (thickness: 1 .mu.m and carrier concentration: 3.times.10.sup.16 cm.sup.-3), an undoped AlGaAs spacer layer 103 (thickness: 10 nm and carrier concentration: 1.times.10.sup.17 cm.sup.-3), an n-type AlGaAs donor layer 104 (thickness: 20 nm and carrier concentration: 1.times.10.sup.18 cm.sup.-3), and an n-type GaAs cap layer 105 (thickness: 10 nm and carrier concentration: 3.times.10.sup.18 cm.sup.-3). Reference numerals 106 and 107 in FIG. 8 denote a gate electrode and source/drain electrodes, respectively.
It should be noted that in the drawings, the term "2DEG" means "2-Dimensional Electron Gas" which is generated at the interface between a barrier and a channel when a hetero junction is formed.
Moreover, FIG. 9 shows a structure of a conventional HFET using a nitride-type III-V group compound semiconductor (U.S. Pat. No. 5,192,987). The illustrated HFET using a nitride-type III-V group compound semiconductor has substantially the same structure as that of the GaAs-type HFET. Specifically, as shown in FIG. 9, the illustrated HFET using a nitride-type III-V group compound semiconductor includes, on an insulating substrate 201 (e.g., a sapphire substrate), an AlN low-temperature grown buffer layer 202 (thickness: 20 nm), a GaN buffer layer 203 (thickness: 2 .mu.m and carrier concentration of 8.times.10.sup.16 cm.sup.-3), an AlGaN donor layer 204 (thickness of 20 nm and carrier concentration of 1.times.10.sup.18 cm.sup.-3), a gate electrode 205, and source/drain electrodes 206, and uses GaN as a channel material.
FIG. 10 shows a reverse structure HFET (Electronics Lett., Vol. 31, No. 22, (1995) pp. 1951-1952). As shown in FIG. 10, the reverse structure HFET includes, on an insulating substrate 301 made of sapphire and the like, an AlN low-temperature grown buffer layer 302 (thickness: 20 nm), a GaN buffer layer 303 (thickness: 3 .mu.m), an AlN barrier layer 304 (thickness: 3 nm), and a GaN channel layer 305 (thickness: 100 nm). Reference numerals 306 and 307 in FIG. 10 denote a gate electrode and source/drain electrodes, respectively.
An electron mobility of GaN, which is conventionally used as a constituting material of a channel layer, is about 200 cm.sup.2 /Vs when its carrier concentration is about 1.times.10.sup.18 cm.sup.-3 and about 400 cm.sup.2 /Vs when its carrier concentration is about 1.times.10.sup.17 cm.sup.-3. This electron mobility is about one order of magnitude greater than that of other wide-band gap materials such as SiC, but about one order of magnitude smaller than that of GaAs used in a GaAs-type HFET.
In the case of the GaAs-type HFET, as described in Japanese Laid-Open Publication No. 63-161678, an InGaAs mixed crystal which has a larger mobility as compared with GaAs can be inserted to an interface of AlGaAs and GaAs as a channel material. Thus, it was considered that the similar method (i.e., insertion of InGaN) would be available for a nitride-type semiconductor device. In contrast to such expectation, however, in the case of the nitride-type III-V group compound semiconductor device, satisfactory crystallinity and/or flatness can not be obtained in the inserted InGaN mixed crystal, so that an electron mobility does not always become larger. Thus, the effect obtainable by an InGaAs channel layer in the GaAs-type HFET cannot be expected by the insertion of InGaN crystal.